The Tiffen 2x3" Yellow 12 filter is commonly used for a noticeable increase in contrast with black and white films. When your black and white images are flat, throw on a contrast filter.
Contrast in black and white is generally controlled by yellow, orange and red filters with red producing the greatest contrast. Moderate changes can be accomplished with yellow which can, generally, bump up the contrast just enough to make the difference between a "muddy" print and an acceptable image.
Especially useful for clear contrast between blue sky with clouds and foreground
Recommended as a basic filter for use with Kodak Aero Ektachrome Infrared film